Before the 1970s, cameras that operated with a flash used flash bulbs. Unlike electronic flashes which fire seemingly instantaneously, flash bulbs had a slight delay between being triggered and flashing. The reason for this delay was that the filament within the flash bulb needed a few milliseconds to heat up before illuminating. To compensate for this, the camera manufacturers built in a (typically) mechanical shutter delay that allowed the shutter and flash to both occur simultaneously. The length of this delay is dependent on the type of flash bulb used, since all of the bulbs have slightly different response times. For instance, the m-series flash bulbs used by a number of classic cameras (such as the Polaroid Land Camera) had a .026 second (26 millisecond) delay. This common delay is referred to as m-sync because it means the camera was synchronized to work with m-series flash bulbs.
If you have any technical aptitude, it should now be very apparent why an electronic flash won't work with older camera. Basically, when you press the shutter, it will trigger the electronic flash, which will immediately illuminate, and then a few milliseconds later, the shutter will open. By the time the shutter has opened, the flash has already happened and is either finished or greatly decayed. This results in not enough light reaching the film. In more advanced cameras that have light sensors to detect the flash (like the Polaroid Land Camera), this will result in the exposure always being off, as the flash will always happen before the sensor has the chance to register it.
The fix for this timing problem is relatively simple. Since the shutter built into the camera can not be sped up, the electronic flash needs to be slowed down. Fortunately, the circuit for doing this is both pretty simple and pretty small. This is good because it means that it can be easily squished into a cumbersome old electronic flash unit.
I am calling this an m-sync hack because I am using it to work with an m-sync camera (26 millisecond delay). However, I have built the circuit with a trim pot such that you can set a delay between 0 and 51 milliseconds. This should allow you to use it with just about any camera that operates on a flash bulb.